1. Field of the Invention
The invention relates generally to micromachined structures and, more particularly, to micromachined structures in which a first frame is coupled to a plate or to a second frame by diametrically opposed torsion bars that permit rotation of the plate or second frame with respect to the first frame about a longitudinal axis of the torsion bars.
2. Description of the Prior Art
A micromachined structure in which a first frame is coupled to a plate or to a second frame by diametrically opposed torsion bars that permit rotation of the plate or second frame with respect to the first frame about a longitudinal axis of the torsion bars is useful for various different purposes. Practical uses for such micromachined structures include optical beam scanners, gyroscopes, flow meters, and profilometer and/or atomic force microscope ("AFM") heads, etc.
U.S. Pat. No. 5,629,790 entitled "Micromachined Torsional Scanner" that issued May 13, 1997, ("the Torsional Scanner patent") describes in detail torsional micro-scanners which may be used for optical beam scanning or beam deflection. Similar structures are also described in U.S. Pat. No. 5,488,863 entitled "Monolithic Silicon Rate-Gyro With Integrated Sensors" that issued Feb. 6, 1996 ("the Rate Gyro patent") for micromachined gyros. The Torsional Scanner patent and the Rate Gyro patent, which are incorporated herein by reference, both describe fabricating stress free plates yielding optically flat surfaces, and metrological grade torsional hinges that provide high quality torsional oscillators.
The Torsional Scanner patent and the Rate Gyro patent also describe incorporating torsion sensors into the torsional hinges, and the use of such torsion sensors to make the devices self oscillating at the devices inherent mechanical resonance frequency. In many applications, the mechanical resonance frequency provides a reference frequency from which other timing parameters are derived for a system that employs the torsional oscillator. By appropriately selecting parameters for the torsional oscillators, and then controlling such parameters during photo-lithographic fabrication, the resonance frequency can often be controlled to within a fraction of one percent of the chosen frequency. For many applications, his degree of frequency control and selection is adequate.
However for some applications such as display devices, it becomes necessary to match the resonance frequency of the torsional oscillator precisely to a predetermined external frequency. For example, the scanning mirror may be used in a display device, and needs to be synchronized exactly to an externally specified raster frequency. Since many of the oscillators that are envisioned for use in displays have a very high Q, it may be impossible to fabricate quantities of torsional oscillators having a specified resonance frequency with high yield. Similarly, for certain applications a micromachined torsional oscillator may exhibit an excessively high Q. Analogously, widespread commercial application of micromachined torsional scanners for display or imaging purposes also requires that the scanner itself be mechanical rugged, i.e. not fracture easily upon receiving an impact.